A series of bimagnetic heterostructured nanocrystals having an antiferromagnetic NiO core and a ferrimagnetic Mn x Ni1–x O and/or FiM Mn3O4 island nanophase overgrowth has been synthesized under varying aqueous solution pH conditions. The two-step self-assembly process employs a thermal decomposition method to synthesize NiO nanoparticles, followed by growth of the Mn x Ni1–x O and/or Mn3O4 nanophase over the NiO core using hydrothermal synthesis at pH values ranging from 2.4–7.0. The environmentally benign hydrothermal process involves pH control of the protonation vs hydroxylation reactions occurring at the nanoparticle surface. TEM analysis and Rietveld refinement of XRD data show that three distinct types of heterostructured nanocrystals occur: NiO/Mn x Ni1–x O core–shell-like heterostructures at the pH of 2.4, mixed NiO/Mn x Ni1–x O and/or/Mn3O4 core-overgrowth structures for 2.4 < pH < 4.5, and predominantly NiO/Mn3O4 core-island structures for pH > 4.5. The magnetic coercivity and exchange bias of the heterostructured nanocrystals vary systematically with the pH of the aqueous solution used to synthesize the samples. The temperature-dependent magnetization and hysteresis loop data are consistent with the nature of overlayer coverage of the NiO core. Our DFT based calculations show that the Mn x Ni1–x O phase has ferrimagnetic properties with a stable spin orientation along the ⟨111⟩ orientation. Furthermore, the calculations show that the magnetic anisotropy constant (K 1) of the Mn3O4 phase is considerably larger than that of the Mn x Ni1–x O phase, which is confirmed by our experimental results. The coercivity and exchange bias field are the largest for the NiO/Mn3O4 core-island nanocrystals, synthesized at a pH value of 5.0, with robust values of nearly 6 kOe and 3 kOe, respectively. This work demonstrates the tunability of hydrothermal deposition, and concomitant magnetic coercivity and exchange bias properties, of Mn x Ni1–x O and/or Mn3O4 nanophase overgrowth over a NiO core with pH, that makes these heterostructured nanocrystals potentially useful for magnetic device, biomedical, and other applications.
Tough organic solar cell (OSC) active layers are necessary to achieve robust, flexible, and stretchable devices. A major challenge is that the brittle small molecule acceptor (SMA) in polymer/SMA bulk heterojunctions results in films prone to mechanical failure. To improve mechanical toughness, we investigate the use of a thermoplastic elastomer (styrene-b-ethylene-butylene-styrene) (SEBS) as an additive in high-performance photoactive layers. We find a consistent transition of all measured parameters [e.g., fracture energy (G c) and power conversion efficiency (PCE)] at a SEBS concentration of 5–10 wt %, which is driven by the miscibility of the SEBS. We use this insight to optimize the SEBS loading for PCE and toughness. Optimized OSCs are found to increase G c significantly with a marginal decrease in PCE, resulting in a record G c·PCE metric, considering all OSC photoactive layers. The pronounced miscibility–function relationship provides a framework to optimize elastomer addition in OSCs for performance and toughness.
A semitransparent shape memory polymer (SMP):silver nanowire (AgNW) composite is demonstrated to be capable of low-temperature actuation, thus making it attractive for wearable electronics applications that require intimate contact with the human body. We demonstrate that the SMP:AgNW composite has tunable electrical and optical transparency through variation of the AgNW loading and that the AgNW loading did not significantly change the mechanical behavior of the SMP. The SMP composite is also capable of electrical actuation through Joule heating, where applying a 4 V bias across the AgNWs resulted in full shape recovery. The SMP was found to have high strain sensitivity at both small (<1%) and large (over 10%) applied strain. The SMP could sense strains as low as 0.6% with a gauge factor of 8.2. The SMP composite was then utilized as a touch sensor, able to sense and differentiate tapping and pressing. Finally, the composite was applied as a wearable ring that was thermally actuated to conformably fit onto a finger as a touch sensor. The ring sensor was able to sense finger tapping, pressing, and bending with high signal-to-noise ratios. These results demonstrate that SMP:AgNW composites are a promising design approach for application in wearable electronics.
The interaction between the itinerant carriers, lattice dynamics, and defects is a problem of long-standing fundamental interest for developing quantum theory of transport. Here, we study this interaction in the compositionally and strain-graded AlGaN heterostructures grown on AlN substrates. The results provide direct evidence that a dimensional crossover (2D–3D) occurs with increasing temperature as the dephasing scattering events reduce the coherence length. These heterostructures show a robust polarization-induced 3D electron gas and a metallic-like behavior down to liquid helium temperature. Using magnetoresistance measurements, we analyze the evolution of the interaction between charge carriers, lattice dynamics, and defects as a function of temperature. A negative longitudinal magnetoresistance emerges at low temperatures, in line with the theory of weak localization. A weak localization fit to near zero-field magneto-conductance indicates a coherence length that is larger than the elastic mean free path and film thickness (lφ>t>lel), suggesting a 2D weak localization in a three-dimensional electron gas. Our observations allow for a clear and detailed picture of two distinct localization mechanisms that affect carrier transport at low temperature.
We have developed a novel set of (Mn3O4 and/or MnxNi1-xO)/NiO heterostructured nanocrystals (HNCs) that show promise for multifunctionality. A two-step procedure was used to synthesize our HNC samples. Thermal decomposition was first used to synthesize NiO core nanoparticles whereas hydrothermal synthesis under varying pH conditions was subsequently used to produce overgrowth of a MnxNi1-xO shell and/or Mn3O4 islands on the NiO core. In this work, we report on the investigation of the defects and surface/interface chemistry of our HNC samples using x-ray photoelectron spectroscopy (XPS). The results from a detailed analysis of the Mn 2p XPS spectra show that the Mn 3+ :Mn 2+ ratio increases with increasing pH value used in synthesis of the samples. This is consistent with a trend toward predominance of Mn3O4 islands at higher pH values, a predominance of MnxNi1-xO shell in the lowest pH values and a mixture of both for intermediate pH values. A reduction of the satellite features of the Ni 2p XPS with increasing pH of the synthesis medium is attributed predominantly to surface/interface defects of the HNCs. Fitting of the O 1s XPS spectra shows that Ni-OH and Mn-OH are likely the dominant contribution to the lateral peak whereas defects such as oxygen in an oxygen deficient environment and/or oxygen vacancies comprise a smaller contribution. Analysis of both the Ni 2p and O 1s XPS measured from our samples shows evidence only for a Ni 2+ chemical environment (i.e., negligible Ni 3+ ) in octahedral coordination consistent with a rocksalt structure and well-ordered NiO or MnxNi1-xO nanomaterial. The presence of point defects and the nature of the surface/interface chemistry as determined using XPS suggests that our HNC samples may also be suitable for heterogeneous catalysis applications.
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